LC devices are based on the alignment of LC molecules on substrates that are sandwiched together to form the LC cell. The interaction of LC with the surface of the substrate is of paramount importance for several reasons. Among the most important ones are A—the necessity to have a returning force (to enable the “free” relaxation) and B—to provide a pretilt angle to trigger a uniform reorientation direction.
The traditional way of achieving the desired alignment of LC molecules was (and still is) the mechanical rubbing of the surface of the substrate [T. Ito and K. Nakanishi. Regularity and narrowness of the intervals of the microgrooves on the rubbed polymer surfaces for LC alignment. In SID International Symposium Digest of Technical Papers, Vol XXIII, pages 393-396, Boston, Mass., USA, May 1992. SID.]. While the substrate itself may be processed in this way to achieve LC alignment, very often, specific alignment layers are first coated on the inner surface of the substrate (carrying already a transparent conductive layer, such as indium tin oxide or ITO), which are then rubbed, FIG. 1 (Prior art). A classical example of such layer is the polyimide polymer family. As it can be easily imagined, the rubbing process is not reliable and may damage the cell and create non uniformities and dust. That is why intensive efforts are devoted to develop non-contact alignment methods of LC alignment. Among others. we should note the photo alignment [Gibbons; Wayne M., Sun; Shao-Tang, Swetlin; Brian J. “Process of aligning and realigning liquid crystal media,” U.S. Pat. No. 4,974,941, Dec. 4, 1990; Chigrinov; Vladimir G., Kozenkov; Vladimir M., Novoseletsky; Nicolic V., Reshetnyak; Victor Y., Reznikov; Yuriy A., Schadt; Martin, Schmitt; Klaus, “Process for making photopolymers having varying molecular orientation using light to orient and polymerize”, U.S. Pat. No. 5,389,698, Feb. 14, 1995] and vacuum deposition (e.g., SiOx, [Kyung Chan Kim, Han Jin Ahn, Jong Bok Kim, Byoung Har Hwang, Hong Koo Baik, Novel Alignment Mechanism of Liquid Crystal on a Hydrogenated Amorphous Silicon Oxide, Langmuir 2005, 21, 11079-11084]). Both approaches are progressing rather rapidly and there are even few companies, which have already announced their use in commercial products (for example, for Projecting Displays using Vertical Aligned LC). However those techniques must be approved yet for large scale, cost effective and reliable manufacturing processes.
So called, polymer-stabilized liquid crystals (PSLC) have been used to “program” the alignment and the reorientation of LC molecules, with (e.g., an electric field) or without the use of external excitation means for that programming [T. Galstian, V. Presniakov, A. Tork, K. Asatryan, Electrically variable focus polymer-stabilized liquid crystal lens, U.S. Pat. No. 7,218,375, May 15, 2007]. However, the material system and the programming method used there did not allow the creation of “programmed surfaces” but let the created polymer network to “float” in the volume of the cell, FIG. 2 (Prior art). This reduces the stability of the structures obtained and also creates volume aggregation of polymer and light scattering on director (average orientation of long molecular axes) orientation defects formed around those aggregations [V. V. Presnyakov; T. V. Galstian, Light Polarizer Based on Anisotropic Nematic Gel with Electrically Controlled Anisotropy of Scattering, Molecular Crystals and Liquid Crystals, Volume 413, Issue 1, 2004, pages 545-551].
A further approach to creating an alignment layer with a desired pre-tilt angle involves using a dual polymer composition having vertical and horizontal components that are then baked or rubbed to achieve different uniform pre-tilt angles. See for example the article by Karen E. Vaughn, Matthew Sousa, Daeseung Kang, and Charles Rosenblatt, “Continuous control of liquid crystal pretilt angle from homeotropic to planar”, APPLIED PHYSICS LETTERS 90, pp. 194102 194102-1, 2007, and the article by Fion S. Yeung, Jacob Y. Ho, Y. W. Li, F. C. Xie, Ophelia K. Tsui, P. Sheng, and H. S. Kwoka, “Variable liquid crystal pretilt angles by nanostructured surfaces”, APPLIED PHYSICS LETTERS 88, pp. 051910-1-051910-3, 2006.
In the known art, pretilt angles are typically between 1 to 6 degrees from the planar surface of the alignment layer. Since the electric field acts on the dipole of the LC molecules, the torque on the LC molecules below 6 degrees is quite weak. This torque becomes greater when the molecules begin to show a greater angle. Thus a LC cell with a 4 degree pretilt can respond better to the control field than a cell with a 1 degree pretilt, and typically consume less electrical power.